What is a plaster base?
A plaster base is a layer of material placed between the background and the plaster so the mortar can key into it mechanically. You use one wherever the background itself is too smooth, too absorbent, too weak or too mixed in nature to be plastered directly.
Without a base, the plaster only grips on the suction and roughness of the background. On smooth concrete, on timber, on plastic, or across the junction between two different materials, that is not enough — the coat debonds, bulges or cracks.
Common types
- Expanded metal lath (rib lath). Galvanised or stainless sheet slit and stretched open. Strong, stiff and the classic choice for plaster over timber or steel backgrounds.
- Glass fibre reinforcing mesh. Alkali-resistant woven glass fibre bedded into the first wet coat. This is standard on external wall insulation systems and for bridging board joints.
- Reed matting. The traditional answer in conservation work and with lime and clay plasters: reed wired to a ceiling or framework. Vapour open, dimensionally stable and historically correct.
- Wood wool cement board. A rigid board that acts as base, insulation and acoustic absorber at once. Widely used on ceilings and on backgrounds with large tolerances.
- Profiled insulation boards for internal insulation with a plaster finish.
When you really need one
- Material junctions. Anywhere plaster runs across the joint between two different backgrounds — concrete against brickwork, calcium silicate against aerated concrete — the coat will almost always crack without reinforcement.
- Over chases. A filled service chase behaves differently from the surrounding wall; run a mesh strip over the full chase plus at least 100 mm lap either side.
- At corners and reveals of window and door openings, where stresses converge.
- On timber and softboard, which move with moisture.
An important detail: never leave glass fibre mesh hanging dry on the background. It belongs in the mortar, roughly in the outer third of the coat thickness — apply mortar first, comb the mesh in, then close up. With metal lath the reverse applies: it is mechanically fixed, and the mortar is pressed firmly through so it keys behind the lath.
Related terms
- Plaster coat
- Wood wool cement board
- Render
- Mortar
- Aerated concrete
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